1. Field of the Invention
The invention relates to method and apparatus for characterizing residual stress in ferromagnetic members and more particularly to such method and apparatus for distinguishing between residual stresses resulting from compression and tension forces in ferromagnetic test members.
2. Description of the Prior Art
Residual stress characterization in ferromagnetic members such as steel load bearing structural beams and girders, wheels, gears and the like is a difficult and critical challenge. While a magnetoacoustic (MAC) method for detecting the presence of residual compression without requiring calibration standards is known, this technique is not capable of detecting the presence of residual tension in a ferromagnetic member and thus characterizing or distinguishing between the two distinct forms of residual stress.
Another technique generally known as magnetoacoustic emission (MAE) (as shown in U.S. Pat. No. 4,408,160) is capable of detecting the presence of residual stress in ferromagnetic materials resulting from both compression and tension forces without the capability of distinguishing between the two.
Numerous other techniques for sensing residual stress in ferromagnetic members are also generally known. For instance, x-ray diffraction is a method capable of quantitatively characterizing the surface residual stress state in metallic objects while the ultrasonic birefringence technique, in principle, is capable of measuring bulk residual stress in solid materials.
However, while x-ray diffraction is the only quantitative method known to the inventors that senses the amplitude of residual stress, a major disadvantage of this technique is that its use is limited to a shallow surface region of metallic objects and the surface preparation necessary to obtain reliable data.
The ultrasonic birefringence technique is in general extremely sensitive to the intrinsic structural properties of members examined, i.e., textures in polycrystals. The effect of texture on the observed birefringence is comparable or even larger than that due to residual stress. Thus this technique requires the use of ultrasonic shear waves with their polarization perpendicular to each other. In practical application it is impossible to maintain the constant mechanical coupling between the ultrasonic transducer and the test object for the two angular positions of the transducer.
The Barkhausen technique as described in the aforementioned U.S. Pat. No. 4,408,160 measures discontinuity in magnetization as a form of a magnetic acoustical noise signal using a pickup coil as a sensor. A disadvantage of this method is that when it is necessary to use an external magnetic core to induce a strong magnetic flux density in a test object, the magnetic noise signal from the core interferes with the acoustical emission signals produced in the test objects. Further there is no way to distinguish or characterize the residual stress as to its cause--forces in compression or tension on the test object.